(1) Field of the Invention
This invention relates to semi fuel electrochemical cells used for undersea vehicle propulsion. Specifically, this invention relates to a system and a method for solid storage and subsequent dissolution of electrolytes/catholytes used in semi fuel electrochemical cells used for undersea vehicle propulsion. More specifically, this invention relates to a system and a method for solid storage and subsequent dissolution of electrolytes/catholytes, wherein a solid medium is provided comprising electrolytes/catholytes in a solid form and includes sodium peroxide particles suspended in a matrix of potassium superoxide and/or sodium hydroxide, wherein the matrix controls the dissolution and hydrolysis rate of the solid catholyte by allowing for a dissolution only from a controlled surface of the solid medium, and wherein the matrix itself is dissolved and hydrolyzed and the products of this dissolution and hydrolysis are usable by the semi fuel cell.
(2) Description of the Prior Art
Primary batteries employing caustic electrolytes with hydrogen peroxide as the cathode species have been under development by the Navy and other laboratories since the 1980s. The aluminum-hydrogen peroxide semi fuel cells have applications in torpedoes, unmanned undersea vehicles (“UUVS”) and other undersea craft that utilize electric energy. Emphasis has been placed on cost reduction in utilizing hydrogen peroxide as the cathode as opposed to earlier silver oxide cathodes. In order to achieve high energy storage densities it has been necessary to consider storing all reactants in concentrated form. Limiting factors in the development of the semi fuel cell systems, however, are the safety issues associated with liquid hydrogen peroxide and the total mass of hydrogen peroxide that would be required to be stored for a given mission.
Research has been conducted in an effort to avoid the problems related to liquid hydrogen peroxide as a catholyte. For example, use of oxygen as the cathode species employing an oxygen reduction cathode membrane has been investigated, developed and demonstrated for use undersea (with bottled oxygen or oxygen candles) and on land (using air as the oxygen source). However, there is a need for higher energy densities than those that have been achieved by these methods. Additionally, as with concentrated hydrogen peroxide, there are also safety concerns associated with each of these systems.
Other forms of concentrated catholyte for fuel cell systems that have been investigated for high energy storage densities are catholytes delivered to catholyte solutions from solid form, for example, powders or pellets. However, these forms for delivering catholytes render control of the dissolution rate of the solid catholyte composition into the aqueous electrolyte difficult. Although catholyte rate delivery can be attained, to some degree, by adjusting the geometrical size and shape of the catholyte composition, the degree of control is too imprecise to properly control the dissolution of the catholyte when its shape and size varies during its lifetime. A related problem is posed by dissolution rate sensitivity of solid catholytes to the flow rate of the catholyte solution.
Attempts to solve the problems associated with solid electrolytes and the control of their dissolution and hydrolysis have been made. For example, U.S. Pat. No. 5,399,444 to Smith discloses a device that operates based on the stated discovery that the concentration of an electrolyte in solution can be maintained even when the electrolyte is being consumed continuously or intermittently by utilizing an electrolyte delivery system based upon internal osmotic pressure. The device employs salts as solid electrolytes, for example sea salt and zinc chloride, or bases such as lithium hydroxide, sodium hydroxide, and potassium hydroxide or mixtures thereof. Particles of electrolytes are coated with a semi-permeable coating. Water is imbibed through the coating and dissolves the water-soluble electrolyte, thereby creating an osmotic pressure that causes the saturated solution of the electrolyte to be pumped out the micro-passageways in the film coating. By adjusting the relative amount of electrolyte solution to solid electrolyte in the delivery, one can maintain the concentration of the electrolyte in solution relatively constant during the use of the electrolyte solution in an electrochemical apparatus.
U.S. Pat. No. 5,529,707 to Kejha discloses solid composite polymeric electrolytes that are made by mixing alkali metal trifluoromethanesulfonate and polyethylene oxide to which mixtures of lightweight non-conductive inorganic fillers such as oxides or peroxides of lithium, magnesium, and sodium have been added with co-solvents of esters and ethers. Solidification of the electrolyte is achieved by the presence of the alkali metal trifluoromethanesulfonate and by partial evaporation of the lower boiling point ether or ester. These electrolytes can be used as solid, semi-solid or liquid state polymer electrolytes for batteries and other electrochemical devices.
None of the above prior art address the problems related to catholytes where the cathode species is hydrogen peroxide obtained by dissolution of caustic oxides, peroxides, or superoxides and which are an alternative to liquid hydrogen peroxide and provide the required high energy densities for semi fuel cells in underwater vehicles. Thus, powdered or crystalline forms of these peroxides, oxides, and superoxides dissolve rapidly in water with an exothermic reaction. It is necessary to control the hydrolysis rate of these solid catholytes to avoid decomposition of hydrogen peroxide as the heat that is generated during the hydrolysis destabilizes hydrogen peroxide before it reaches the fuel cell.
Consequently there is a need in the art for an inexpensive system and method that utilizes solid catholytes such as oxides, peroxides and superoxides of alkaline metals in which the rate of hydrolysis is controlled to limit the production of cathode species (H2O2) to the rate at which they are consumed in the semi fuel cell.